I found really inexpensive silver satin tablecloths (here), and I made simple centerpieces out of white hydrangeas and roses with some green accent flowers. I had planned on using green orchids, but couldn't find them. The smaller filler flowers worked just fine.
For the seating cards, I found small apples at Michael's and I cut out little leaves with everyone's name on them. I punched a hole in each leaf and hung it on an apple. (I am a big fan of seating cards for a formal holiday dinner - I don't have to deal with everyone asking me where to sit while I'm trying to get the food ready, and I can also arrange the seating in a way that ensures good conversations around the entire table).